Multimedia conferencing has become one of many important applications of network communications, especially over the Internet. Multimedia conferencing is preferably carried out in a multicast mode to enable real-time interactions among several participants, where each participant can see video signals and hear sounds of the other participants. To participate in a multimedia conference, a user of a client machine uses a multimedia conferencing component, which may be a component of a Web browser, to place a multicast call to the other participants. The multimedia traffic from all participants is sent to the multicast address associated with the conference, and each client listening on that address is capable of simultaneously receiving and rendering multimedia streams from all the other clients. In this way, the user is able to hear all the other participants and see them simultaneously in different video windows.
Multimedia conferencing using the multicast technology requires that all of the clients participating in the video conference be connected together by a multicast-enabled network, and all of the clients be multicast-capable. There are, however, many network clients that are not able to participate in a network conference in the multicast mode because either they do not have multicast capabilities or they reside on networks that cannot form a multicast connection to the conference server. Due to the large number of such clients, it is desirable to provide ways for them to participate in a multimedia network conference.
One proposed way to allow a client that is not able to join a network conference in the multicast mode to still participate in the conference is to have the client place a point-to-point call to a bridge service that is multicast capable and connected to the multicast network. The bridge service then joins the conference on behalf of the client. During the conference, the audio and video streams generated by the client is unicast to the bridge service. The bridge service functions as a proxy of the client by forwarding conference traffic from this client to the other participants, and to forward conference traffic originating from the other participants to the client.
One problem with the use of a bridge service, however, is that the unicast client is not able to see all the other participants at the same time. While a multicast participant can receive the multiple video data streams from all the other participants, the point-to-point connection between the unicast client and the bridge service is capable of handling and rendering only a single video substream from one participant. In other words, the unicast client can only see one of the other participants at a time. The difficulty is in deciding which video substream should be presented to the unicast client for viewing. Thus, there exists a need for an intelligent way for the bridge service to select one video substream from all the video substreams originated from multiple conference participants for viewing by the client. The resulting user experience should be seamless and adaptive to different scenarios of conference dynamics.